This invention relates to polyvinyl butyral sheet for laminated safety glazings and more particularly to plasticizing such sheet for impact dissipation performance.
Polyvinyl butyral (PVB) polymer for use as optically transparent, impact-dissipating sheet in layered laminated safety glazings is well known. Since the glass transition temperature (hereinafter "T.sub.g ") of unplasticized PVB polymer is too high for it to be a useful elastomer for impact dissipation in these applications, it is also known to reduce such temperature to a useful range by incorporating a plasticizer in such sheet.
Plasticizer admixed in sheet with PVB is not without problems. Depending on volatility, plasticizer can evaporate from the sheet during handling and storage before use. Moreover, in forming the safety glazing elevated temperature on the order of about 150.degree. C. is required to bond the PVB sheet to glass, and significant plasticizer can be lost, particularly along usually unsealed edges of the sheet in the laminate. Perhaps of greater significance are performance problems occurring along the edge of the laminate during its use life which are traceable to the plasticizer. Plasticizer can be displaced from the sheet by atmospheric moisture along unsealed edges in a high humidity environment causing swelling over time. When followed by dry weather, the edge dries and contracts and after many such cycles the unstable edge begins to crack. Moreover, moisture in such high humidity applications can cause a bluish haze in the edge region of the laminate (called edge blush) which is an appearance problem in applications such as windshields where the laminate is exposed to its edge.
It would be desirable to provide soft, rubbery PVB sheet capable of absorbing impact for safety glazing use without exclusively relying on the presence of plasticizer as above described.